1. Field of the Invention
This invention relates to a method and apparatus for forming glass containers having reinforced inner surfaces when the glass containers are produced by a press-and-blow system.
2. Description of the Prior Art
Prior art methods for shaping glass containers include a blow-and-blow system and a press-and-blow system.
FIGS. 1a-1f are schematic views showing a glass gob and molds at various stages in the shaping process of a glass container shaped by the press-and-blow system. The production process proceeds from the first to the sixth stages depicted successively in FIGS. 1a-1f. In the first stage, a glass gob 2 is charged into a blank mold 1 through a funnel 4, and a plunger 3 is situated in a stand-by position below the gob 2. In the second stage, the gob 2 is disposed inside the blank mold 1 and a baffle 6 is fitted thereover. In the third stage (or pressing stage), the plunger 3 is extended into the gob 2 to form the gob generally into .the shape of a container, such container-shaped mass being called a parison. In the fourth stage, this parison 7 is transferred by a neck ring mold 5 to a finish mold 8 which has a bottom plate 9. In the fifth stage, the neck ring mold 5 is removed and the parison 7 is disposed in the finish mold 8. In the sixth stage (or final blowing stage), a blow head 10 is fitted to the finish mold 8, and compressed air a is blown into the parison 7 through the blow head 10 to form a glass container having the desired final shape.
The press-and-blow system has, to date, been the preferred system for shaping lightweight containers because it is capable of easily providing a uniform thickness distribution for the containers. However, in the press-and-blow system the parison is produced by pressing the plunger into the gob in the blank mold. Accordingly, the inner surface of the parison tends to have many fine scratches due to contact with the plunger and also its temperature tends to drop.
FIG. 6 shows exemplary values of the inner surface temperature of the parison in the press-and-blow system in comparison with those in the blow-and-blow system. It can be understood from this diagram that a remarkable temperature drop occurs during pressing of the plunger into the gob in the press-and-blow system.
Traces of the fine scratches on the inner surface of the parison remain even after the final blowing stage has been completed. The presence of these fine scratches tends to reduce the inner surface strength of the containers produced. Furthermore, the drop in the inner surface temperature of the parison exerts adverse influence on the reheating time and the molding of the parison.
The blow-and-blow system does not have a pressing step. Therefore, the containers produced thereby have a high inner surface strength. However, such containers do not have a uniform thickness distribution, thereby making it difficult to reduce the weight of the containers.
Accordingly, the press-and-blow system has generally been adopted in order to reduce the weight of the containers. In addition, in order to improve the outer surface strength of the containers, so-called hot-end coating or cold-end coating has been employed. However, there has heretofore been no particular method of retaining or increasing the inner surface strength of the containers.
Thus, in the press-and-blow system, fine scratches on the inner surface of the parison caused by contact between the gob and the plunger and the drop of the inner surface temperature of the parison have remarkably lowered the strength of the inner surfaces of the glass containers produced. Accordingly, a method and apparatus for preventing such troubles have long been anticipated.